Peak horsepower meter

ABSTRACT

A peak horsepower meter for locomotives is provided which enables spot checking of locomotive horsepower during operation. The overall system preferably includes base plates installed on each locomotive which provide first and second signals related to traction motor current and voltage, respectively. A single portable unit which plugs into a socket in each base plate is used to monitor the horsepower for each locomotive. The overall meter circuitry includes a current shunt for producing the first signal and a voltage divider for producing the second signal and an electronic switch for electrically multiplying the two signals together to produce an analog signal related to instantaneous locomotive horsepower, isolation transformers serving to electrically isolate the electronic switch. The analog signal is converted to a peak indication by a circuit including a voltage comparator which compares the analog signal with a signal corresponding to the previous peak and which controls stepping of a stepping motor which drives a digital display device.

United States Patent [1 1 Scott Nov. 20, 1973 PEAK HORSEPOWER METER [75]Inventor: William Michael Scott, Montreal,

Quebec, Canada [73] Assignee: Canadian National Railway Company,Montreal, Quebec, Qanada [22] Filed: Mar. 20, 1972 [21] Appl. No.:235,911

[56] References Cited UNITED STATES PATENTS 2/1972 Calud 73/136 R OTHERPUBLICATIONS Ryerson, Instr. & Control Systems, pp. 95-97; Jan. 1963.

Primary ExaminerAlfred E. Smith AttorneyFetherstonhaugh & Co.

57 ABSTRACT A peak horsepower meter for locomotives is provided whichenables spot checking of locomotive horsepower during operation. Theoverall system preferably includes base plates installed on eachlocomotive which provide first and second signals related to tractionmotor current and voltage, respectively. A single portable unit whichplugs into a socket in each base plate is used to monitor the horsepowerfor each loco motive. The overall meter circuitry includes a currentshunt for producing the first signal and a voltage divider for producingthe second signal and an electronic switch for electrically multiplyingthe two signals together to produce an analog signal related toinstantaneous locomotive horsepower, isolation transformers serving toelectrically isolate the electronic switch. The analog signal isconverted to a peak indication by a circuit including a voltagecomparator which compares the analog signal with a signal correspondingto the previous peak and which controls play device.

15 Claims, 3 Drawing Figures lOb OlO Q 24 The present invention relatesto a peak horsepower meter particularly for use with a diesel electriclocomotive.

BACKGROUND OF THE INVENTION In operating a diesel electric locomotive,it is, of course, desirable to know whether the locomotive is in properrunning condition such that the locomotive is capable of delivering,upon demand, the maximum horsepower output thereof. Most heavy trainsare presently pulled by multiple locomotives in tandem and this makes itdifficult to determine which out of a number of units is not pulling itscorresponding share of the load. Further, to obtain the horsepoweroutput of a particular diesel locomotive it is presently necessary toremove that locomotive from service and put the locomotive through whatis known as a load box" test in a railway shop. The load box simulatesthe traction motor loads and by measuring the current delivered and thevoltage produced, an indication of the electrical horsepower of thelocomotive can be obtained under various locomotive operatingconditions. It should be pointed out that, because of the necessity ofremoving the locomotive from service, the test is performed only at'veryinfrequent intervals and as a result a locomotive may be performingpoorly for an extended period'of time before this fact is discovered.

SUMMARY or THE INVENTION In accordance with the invention, a horsepowermeter is provided which is particularly adapted for use on locomotives.The meter of the invention preferably provides an indication, in digitalform, of the peak horsepower output (per traction motor), of alocomotive during actual operation. In addition, a further output isprovided which produces a voltage proportional to instantaneous"locomotive horsepower.

The meter system of the invention preferably includes a plug-in typemeter unit and an inexpensive base plate installed on each locomotivethe horsepower of which is to'be determined. The base plate includes anoutput socket at which are provided electrical signals corresponding tothe parameters, i.e., traction motor current and voltage, used indetermining peak horsepower, and thus throughfthe use of a single meterunit, a large number of locomotives can be monitored on a spot check"basis. Hence, the meter system of the present invention enables adetermination to be made as to whether a locomotive is developing fullhorsepower between the infrequent load box tests discussed above.

According to a presently preferred embodiment, the horsepower meter ofthe inventor comprises a first means, preferably in the form of acurrent shunt, for producing a first electrical signal related to thecurrent through a traction motor of the locomotive and a second means,preferably in the form of a voltage divider network, for producing asecond signal related to the voltage across that traction motor. Thefirst signal is converted into a further signal having a frequencycorresponding to the traction motor current, which Signal is multipliedelectrically by the second signal to produce a voltage signalfrelatedtoinstantaneous locomotive horsepower. The current shunt and voltagedivider means are electrically isolated from the transistor switch usedin the electrical multiplication operation preferably by means ofisolation transformers connected'in the two branches. The output of thevoltage divider network is chopped and amplified before passing throughthe isolation transformer and rectified after passing therethrough.

The analog signal produced by the transistor switch and associatedamplifier and filter is'converted into a peak horsepower indication by acircuit including a voltage comparator which compares the analog signalwith a signal corresponding to the previous peak indication, a digitalindicator or display device driven by a stepping motor and pulsegenerator means including an electronic switch responsive to the outputof the voltage comparator for controlling stepping of the stepping motorin accordance with that output.

Other features and advantages of the invention will be set forth in orapparent from the detailed description of a preferred embodiment setforth hereinbelow.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of ahorsepower meter in accordance with a presently preferred embodiment ofthe invention showing the portable meter unit detached from and spacedfrom the stationary transducer unit;

FIG. 2 is a schematic block circuit diagram of the current and voltagetransducer and pulse multiplier portion of the electrical system of thehorsepower meter of the invention; and

FIG. 3 is a schematic block circuit diagram of the peak follower portionof the electrical system of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, the peakhorsepower meter system of the invention, which is indicated generallyat 10, basically comprises a meter unit 10a and a base plate 10b. Themeter unit 1011, which is of portable, plug-in design, includes a casingor housing unit 12 which houses a majority of the electrical circuitrydescribed hereinbelow in connection with FIGS. 2 and 3. The meter unit10a is shown in FIG. 1 with the control panel cover removed so as toshow the control panel 14. The control panel 14 includes a digitalindicator 16 which produces a digital indication of the peak horsepoweras well as an output jack or socket 18 which, in operation, provides anoutput voltage proportional to instantaneous locomotive horsepower asdescribed hereinafter. In addition, a pair of control switches 20 and areset button 22 (also described below) are also included on controlpanel 14.

Meter unit 10a includes-a plug (not shown) located on the back thereofwhich is adapted to be plugged into a socket 24 in base plate 10b. Baseplate 10b is of inexpensive design and is adapted to be fixedly mountedor installed in a locomotive to be tested. By installing correspondingbase plates in a series of locomotives, each suchlocomotive can betested using a single meter corresponding to unit 10a. Six input wiresor conductors are required for the overall test unit, four from the 10-comotive traction circuit as discussed hereinbelow and two from'thepower supply, which, in this instance, is provided by two 74 volt D.C.wires. The base plate assembly 10b includes connections to the tractionmotor circuit, connected to an input terminal'indicated at 28,

so that by plugging in unit a, voltage signals.corresponding to therequired tractor motor currents and voltages are available at theoutlets of socket 24. The power supply for unit 12 is also providedatthe outlets of socket 24. The power supply is derived from the 10-;

comotive circuits and is preferably of. the type. described in U.S.application Ser. No. 753,138, filed on Aug. 16, 1968. With the use ofthree separate floating reference potentials it is then possible'toconnect the ground or common of the supply potentials for the horsepowercomputation circuitry described below di- 1 rectly to the frame of thelocomotive to minimize any danger to personnel. This is made possiblethrough the provision of two isolating transformers as discussedhereinafter.

To produce an electrical signal related to the horsepower output of adiesel electric locomotive, it is necessary to measure the currentthrough a traction motor and the voltage across that motor and convertthe values thus obtained the frequency signals which can be processedelectronically. To this end, referring to FIG. 2, a current shunt 30 andvoltage divider 32 are provided. It is noted that the main generator oralternator current could be used but a current shunt providing a measureisolation of this current is not available on most locomotives.Typically, current shunt 30, which is connected in series with atraction motor, produces an output of either 50 or 100 millivolts forl,000 amperes of traction motor current. Control switches of meter 10include a range switch for adapting the meter to the shunt used. Itwill, of course, be appreciated that a shunt of any adequate currentrating can be used and that the values given are merely exemplary.Voltage divider 32 is, as indicated, connected across the traction motorand produces a voltage proportional to the traction motor voltage. Asdescribed hereinafter the current and voltage signals are firstprocessed independently and then combined for further processing.

'As stated, current shunt 30 produces an output voltage signalproportional to the traction motor current, which signal is applied to avoltage to frequency converter 34. Converter 34 is of conventionalconstruction and produces an output signal thefrequency of which isproportional to the traction motor current. The waveform of the outputvoltage to frequency converter 34 is generally that of a repetitive rampvoltage, as is indicated, and this voltage is processed in a pulseamplifier 36 to produce constant duration pulses of convenient durationT indicated at the output of amplifier 36. Because the possibilityexists of having a 1,500 volt potential difference between the currentand voltage sensors, the pulses from amplifier 36 are passed through aniyolation transformer 38. Isolation transformer 38 distorts the waveformof the pulse input thereto, as indicated, and a pulse shape recoverycircuit or pulse shape 40 is provided to restore the pulse train to theoriginal shape thereof. Thus, the circuitry thus-far described producesa pulse train having a frequency proportional to the traction motorcurrentand electrically-isolated from the current shunt 30. In thislatter regard tit is noted in a locomotive, the alternator or maingenerator output is floating" with regard to the frame or chassis.

In addition, the current and'voltage inputs may. have to be taken atopposite ends of 'this output. Since, as described above, the peakhorsepower meter of the invention is mounted to'the locomotive chassisit is imperative that all gthesignal inputs are.isolated from eachother, and from the frame, to avoid injury to personnel.

The output .of' voltage divider 32 is also electrically isolated-fromthe remainder of the circuit. This output I is first converted into anA.C., or pulsating D.C. voltage by means of a D.C. chopper circuit 42.Chopper 42 similarly to the other circuits hereindescribed is ofconventional construction and includes a transistor which is driven intoand out of conduction at a predetermined rate. It should be noted thatthe chopper frequency is not important and that only the amplitude ofthe chopped D.C. voltage V2 is relevant. An A.C. amplifier 44 connectedto the output of chopper 42 provides a low impedance drive for a secondisolation transformer 46. A rectifier 48 connected to the output oftransformer 46 produces a voltage V2 which is equal in magnitude to thevoltage produced by voltage divider circuit 32 but which is electricallyisolated from that circuit. It is noted that the value of the output ofthe rectifier 48 would actually be V2/2 but for the sake of simplicityit is assumed that the signal is suitably amplified somewhere betweenchopper 42 and the output of rectifier 48 such that the rectifier outputis a voltage V2 as stated. I

The output of rectifier 48 is connected to an operational amplifier andfilter 50 through a transistor switch 52. The periods of. conduction andnonconduction of switch 52 are controlled by the pulses from pulse shaperecovery-circuit 40. Thus, the output of transistor switch 32 is a trainof pulses whose amplitude is proportional to the traction motor voltageV2 and whose frequency is proportional to the traction motor current.The D.C.- gain of operational amplifier 50 is adjustable for use withboth 50 and millivolt shunts. Further, capacitors of various values maybe. switched into the feedback path of the operational amplifier 50 tovary the response time between about 1 second to a minute depending onwhether the-meter is used to monitor sustained peak horsepower orinstantaneous horsepower. The term sustained" horsepower is perhaps moreappropriate than instantaneous horsepower, where, as set forth, thesystem is operating in a mode providing a response time of approximatelyone-half minute. It is noted that for measuring sustained peakhorsepower the response should be relatively slow seas to avoid readingsbased on momentary horspower peaks such as occur during transition andwheel slip on some locomotives. The filter in amplifier and filtercircuit 50 performs an averaging function and produces a D.C. voltagewhose magnitude is directly proportional to the product of tractionmotor voltage and tractor motor current, a product which is, of course,proportional to locomotive horsepower.

As shown in FIG. 1 the output of amplifier and filter circuit 50 isconnected to output socket or jack 18 referred to hereinabove, whichoutput, being proportional to instantaneous or sustained locomotive peakhorsepower, can be used to drive a chart recorder or the like. It shouldbe noted that either the current or voltage inputs can be disconnectedand held at some constant .value so that the output at jack 18 becomesan indication of instantaneous current or voltage. To this end, a firsttwo-pole switch 47 is provided which operates, in the second positionthereof shown in dashed lines in FIG. 2, to disconnect the voltage inputto transistor switch ,52 from the motor voltage sensing circuit and to,connect-thisinputto a constant voltage source indicated at 49. In themode, the output at jack 18 is a measure of instantaneous current.Similarly, a second switch 51, in the dashed line position thereof,disconnects the input from the current sensing circuit and connects theinput to the transistor switch 52 to a constant current source indicatedat 53, jack 18 thus providing an output which is a measure of theinstantaneous voltage. However, as discussed above, the principle usecontemplated for the system of the invention is to provide a peakhorsepower indication.

Referring to FIG. 3, the peak follower portion of the electrical systemis shown. An1 operational amplifier 54 is connected as a voltagecomparator with one input connected to receive the sustained horsepoweroutput signal of circuit 50 as indicated. The second input receives afeedback signal proportional to the previous sustained peak horsepowerdisplayed. The output of comparator amplifier 54v is connected to anelectronic switch 56 which. controls the transmission. of pulse frompulse generator 58 to a stepping motor 60. Stepping motor 60 drives ashaft 62. for peak horsepower display device or indicator 16, a IO-turnpotentiometer 64 being mounted. on shaft 62 to provide the feedbacksignal discussed. above. Pulse generator 58. continuously attempts topulse stepping motor 60in the forward direction but the signalstherefrom pass. to motor 60 only when electronic switch 56- isconducting which is in turn determined by comparator amplifier 54. Ifthe instantaneous analog horsepower signal is greater than the previousmaximum horsepower being displayed by indicator 16, the comparatoramplifier actuates the electronic switch 56sothat pulses from pulsegenerator 58 cause stepping of motor 60 until the feedback signal frompotentiometer 64 is equal to or slightly greater than the analoghorsepower signal from circuit 50. At this point, the drive to steppingmotor 60 is deactuated, switch 56 beingnon-conducting, and a new peakhorsepower indication is displayed by indicator 16. Should it be desiredto reset the display to zero, the reset pushbutton' 22 referred toabove, and connected between pulse generator 58' and motor 60, isdepressed until stepping motor 62' is driven in the reverse direction asufficient amount so that indicator 16 reads zero.

The switches 20 mentioned above are used to scale the device for, in anexemplary embodiment, 50 millivolt or 100 millivolt, 1,000 amperecurrent shunt operation and to slow the response of the meter to preventindications of momentary peaks such as occur during transition or wheelslip. on some locomotives.

Considering the operational characteristics of the meter system, a testsystem in accordance with the invention consumed about 60 watts from the74 volt lines. During extreme temperature tests, the device has beenshown to be capable of an accuracy of i horsepower in ambienttemperatures ranging between -25 to +140F, for horsepower outpus peraxle of from 250 to 1,000 horsepower. Readings obtained during actualroad tests have indicated that the results produced are within 1 percentof those obtained by calculation from load box meters.

Although the invention has been described with respect to an exemplaryembodiment thereof, it will be understood by those skilled in the artthat variations and modifications in this embodiment may be effectedwithout departing from the scope and spirit of the invention.

I claim:

1. A peak horsepower meter for locomotives comprising means responsivethe current flow through a traction motor of said locomotive forproducing a first electrical output signal in accordance with thiscurrent flow, means responsive to the voltage across said traction motorfor producing a second electrical signal in accordance with thisvoltage, means for multiplying said first and second signals together toproduce a further electrical signal in accordance with the horsepoweroutput of said traction motor, means for isolating the multiplying meansfrom said traction motor current responsive means and said tractionmotor voltage responsive means and peak horsepower indicating means forconverting the output of said multiplyingmeans into an indication ofpeak horsepower and means for displaying said indication, saidconverting means including comparator means for comparing said furtherelectrical signal produced by said multiplying means with a signalcorresponding to the previous peak horsepower indication.

2. A horsepower meter as claimed in claim 1 wherein said currentresponsive means comprises current shunt means for producing an outputvoltage in accordance with the current flow through the traction motor,said meter further comprising voltage to frequency converter meansconnected to the output of said current shunt means for converting saidvoltage signal into a further signal having a frequency proportional tosaid voltage.

3. A horsepower meter as claimed in claim 2 wherein said isolating meanscomprises an isolation transformer connected betweensaid voltage tofrequency converter means and said multiplying means.

4. A horsepower meter as claimed in claim 2 further comprising pulseamplifier means connected to the output of said voltage to frequencyconverter means for shaping that output and pulse shape recovery meansconnected to the output of said isolationtransformer for shaping theoutput signal therefrom.

5. A horsepower meter as claimed in claim 3 further comprising choppermeans connected to the output of said traction motor voltage responsivemeans for converting said second electrical signal into a time varyingperiodic signal, said isolationg means further comprising a secondisolation transformer connected between said chopper means and saidmultiplying means, said meter further comprising rectifier meansconnected to the output of said second isolation transformer.

6. A meter as claimed in claim 5 wherein said chopper means comprises atransistor chopper, said meter including an AC. amplifier connected tothe output of said chopper.

7. A meter as claimed in claim 5 wherein said multiplying meanscomprises a transistor switch including a first input connected to theoutput of said rectifier and a gating input connected to receive asignal having a frequency corresponding to the frequency of the outputof said converter means such that the periods of conduction of saidtransistor switch correspond to said frequency.

8. A meter as claimed in claim 7 wherein said multiplying means furthercomprises amplifier and filter means connected to the output of saidtransistor switch.

9. A meter as claimed in claim 1 wherein said peak horsepower indicatingmeans further comprises a stepping motor for driving said display means,a pulse generator for controlling stepping of said stepping motor and anelectronic switch responsive to said comparator means and connectedbetween said pulse generator and said stepping motor, for controllingthe passage of pulses from said pulse generator to said stepping motor.

10. A meter as claimed in claim 9 wherein said display means includes apotentiometer driven by said stepping motor for producing said signalcorresponding to the previous peak horsepower indication.

11. A peak horsepower measuring system for an electric locomotivecomprising stationary means, individual to each locomotive thehorsepower of which is to be measured, for producing a first signalrelated to the current through a traction motor of the correspondinglocomotive and a second signal related to the voltage across saidtraction motor and a portable meter unit including means for detachablyconnecting said unit to said stationary means, said portable unitcomprising means for multiplying together signals related to said firstand second signals to produce a resultant signal related to theinstantaneous horsepower of the corresponding locomotive, means forelectrically isolating said multiplying means from said stationarymeans, and peak horsepower indicating means for converting saidresultant signal into an indication of peak horsepower, said peakhorsepower indicating means comprising comparator means for comparingthe resultant signal produced by said multiplying means with a signalcorresponding to the previous peak horsepower indication and displaymeans for displaying the peak horsepower indication in accordance withthe output of said comparator means.

12. A peak horsepower measuring system as claimed in claim 11 whereinsaid means for converting said resultant signal into an indication ofpeak horsepower further comprises stepping motor means for driving saiddisplay means and pulse generator means responsive to the output of saidcomparator means for controlling stepping of said stepping motor.

13. A peak horsepower measuring system as claimed in claim 1 1, whereinsaid means for producing said first and second signals includes acurrent shunt for producing said first signal and voltage divider meansfor producing said second signal, said portable unit including voltageto frequency converter means connected, in operation, to the output ofsaid current shunt for converting the voltage output of said currentshunt into a signal having frequency related to the current through thesaid traction motor.

14. A system as claimed in claim 11 further comprising means fordisconnecting the said first signal from the input to said multiplyingmeans and holding the current input to said multiplying means constantso as to produce an output related to the instantaneous voltage acrossthe traction motor.

15. A system as claimed in claim 11 further comprising means fordisconnecting the said second signal from the input to said multiplyingmeans and holding the voltage input to said multiplying means constantso as to produce an output related to the instantaneous current throughthe traction motor.

1. A peak horsepower meter for locomotives comprising means responsivethe current flow through a traction motor of said locomotive forproducing a first electrical output signal in accordance with thiscurrent flow, means responsive to the voltage across said traction motorfor producing a second electrical signal in accordance with thisvoltage, means for multiplying said first and second signals together toproduce a further electrical signal in accordance with the horsepoweroutput of said traction motor, means for isolating the multiplying meansfrom said traction motor current responsive means and said tractionmotor voltage responsive means and peak horsepower indicating means forconverting the output of said multiplying means into an indication ofpeak horsepower and means for displaying said indication, saidconverting means including comparator means for comparing said furtherelectrical signal produced by said multiplying means with a signalcorresponding to the previous peak horsepower indIcation.
 2. Ahorsepower meter as claimed in claim 1 wherein said current responsivemeans comprises current shunt means for producing an output voltage inaccordance with the current flow through the traction motor, said meterfurther comprising voltage to frequency converter means connected to theoutput of said current shunt means for converting said voltage signalinto a further signal having a frequency proportional to said voltage.3. A horsepower meter as claimed in claim 2 wherein said isolating meanscomprises an isolation transformer connected between said voltage tofrequency converter means and said multiplying means.
 4. A horsepowermeter as claimed in claim 2 further comprising pulse amplifier meansconnected to the output of said voltage to frequency converter means forshaping that output and pulse shape recovery means connected to theoutput of said isolation transformer for shaping the output signaltherefrom.
 5. A horsepower meter as claimed in claim 3 furthercomprising chopper means connected to the output of said traction motorvoltage responsive means for converting said second electrical signalinto a time varying periodic signal, said isolationg means furthercomprising a second isolation transformer connected between said choppermeans and said multiplying means, said meter further comprisingrectifier means connected to the output of said second isolationtransformer.
 6. A meter as claimed in claim 5 wherein said chopper meanscomprises a transistor chopper, said meter including an A.C. amplifierconnected to the output of said chopper.
 7. A meter as claimed in claim5 wherein said multiplying means comprises a transistor switch includinga first input connected to the output of said rectifier and a gatinginput connected to receive a signal having a frequency corresponding tothe frequency of the output of said converter means such that theperiods of conduction of said transistor switch correspond to saidfrequency.
 8. A meter as claimed in claim 7 wherein said multiplyingmeans further comprises amplifier and filter means connected to theoutput of said transistor switch.
 9. A meter as claimed in claim 1wherein said peak horsepower indicating means further comprises astepping motor for driving said display means, a pulse generator forcontrolling stepping of said stepping motor and an electronic switchresponsive to said comparator means and connected between said pulsegenerator and said stepping motor, for controlling the passage of pulsesfrom said pulse generator to said stepping motor.
 10. A meter as claimedin claim 9 wherein said display means includes a potentiometer driven bysaid stepping motor for producing said signal corresponding to theprevious peak horsepower indication.
 11. A peak horsepower measuringsystem for an electric locomotive comprising stationary means,individual to each locomotive the horsepower of which is to be measured,for producing a first signal related to the current through a tractionmotor of the corresponding locomotive and a second signal related to thevoltage across said traction motor and a portable meter unit includingmeans for detachably connecting said unit to said stationary means, saidportable unit comprising means for multiplying together signals relatedto said first and second signals to produce a resultant signal relatedto the instantaneous horsepower of the corresponding locomotive, meansfor electrically isolating said multiplying means from said stationarymeans, and peak horsepower indicating means for converting saidresultant signal into an indication of peak horsepower, said peakhorsepower indicating means comprising comparator means for comparingthe resultant signal produced by said multiplying means with a signalcorresponding to the previous peak horsepower indication and displaymeans for displaying the peak horsepower indication in accordance withthe output of said comparator means.
 12. A peak horsepower measuringsystem as claimed in claim 11 wherein said means for converting saidresultant signal into an indication of peak horsepower further comprisesstepping motor means for driving said display means and pulse generatormeans responsive to the output of said comparator means for controllingstepping of said stepping motor.
 13. A peak horsepower measuring systemas claimed in claim 11, wherein said means for producing said first andsecond signals includes a current shunt for producing said first signaland voltage divider means for producing said second signal, saidportable unit including voltage to frequency converter means connected,in operation, to the output of said current shunt for converting thevoltage output of said current shunt into a signal having frequencyrelated to the current through the said traction motor.
 14. A system asclaimed in claim 11 further comprising means for disconnecting the saidfirst signal from the input to said multiplying means and holding thecurrent input to said multiplying means constant so as to produce anoutput related to the instantaneous voltage across the traction motor.15. A system as claimed in claim 11 further comprising means fordisconnecting the said second signal from the input to said multiplyingmeans and holding the voltage input to said multiplying means constantso as to produce an output related to the instantaneous current throughthe traction motor.